Fix bug 1727: fix metallb ip range

Metallb was complaining when indicating an IP range of a single IP.
This commit changes the IP range "<IP>-<IP>" to CIDR format: "<IP>/32"

Change-Id: If879fab2b272927a6a8ffda00f9c49f218224ac0
Signed-off-by: David Garcia <david.garcia@canonical.com>